Introduction to replacing a Lucas voltage regulator

The original regulator fitted to my 1951 Matchless G3Ls was a Lucas type ‘MCR-2’ unit (part number 37144-A) which is also sometimes referred to as the ‘AVR’ (Auto Voltage Regulator).

Charging of the battery has been intermittent since I bought the bike about 6 months ago with the ammeter needle constantly flicking back and forth. After checking that my Lucas dynamo all seemed to be in order (see my guide here) and replacing the reproduction MINDA ammeter with a genuine Lucas one (see here) , I concluded that the problem must be with the regulator.

After removing the old Lucas regulator from the bike, a quick inspection soon revealed the likely cause of my intermittent charging fault and flickering ammeter needle. Several of the soldered connections joining the solenoid coils to the terminals had broken down leaving the ends of the coil wires flapping in thin air, occasionally making contact but mostly stopping the system working properly. I’m not sure whether this was a result of age, overheating or vibration, but I suspect it was probably a combination of all three.

Loose regulator connections

Now the solenoid connections could obviously have been re-soldered back into place quite easily, but the rest of the regulator looked like it was it need of some thorough testing and servicing too and I didn’t have the necessary tools to hand to do this properly. And so I decided that this would need to be replaced and that a modern solid-state electronic device was probably the best way forward for better reliability. The original Lucas unit would be left in place in order to keep things looking as original as possible, but it would be disconnected from the charging system and replaced electrically with a discretely placed modern unit.

The article that follows is a step-by-step guide as to how I went about replacing the original electro-mechanical Lucas MCR-2 regulator with a modern solid-state electronic unit. It includes the following sections:

Step 1 – Choosing a replacement regulator

There are various solid-state regulator units available to replace the original electro-mechanical Lucas units. Most of these are fairly similar both in terms of what they do (regulate the dynamo output voltage!) and their price (£45-55). A few of the different makes and models I’ve come across are as follows:

The manufacturer claims that the unit has good circuit protection (in case I was to accidentally connect it wrongly) and better low-rpm voltage output compared to the competitors “V-reg II” unit. Having zero battery drain back to the dynamo at low speeds is also a useful feature, as is the option to switch to 12 volt operation without needing to change or modify the original 6v dynamo.

Update: After my 6 volt battery failed and I couldn’t get a replacement locally, I recently switched my “DVR2” across to 12 volt operation mode. My guide for how I did it can be found here.

Step 2 – Removing the old regulator

The first thing to do on the bike is to remove the old Lucas regulator unit and the bracket that it is mounted on from the frame. Before working on anything electrical it is always best to remove the battery (or at least to safely disconnect it) to prevent any accidental shorting, especially when you later come to wire in your new regulator unit. It is probably easiest to remove the seat in order to get to the wiring and nuts holding the regulator in place.

Step 3 – Selecting a location to install the new regulator

There a few options available for where to install the new voltage regulator unit. One option is to remove the internals of the old Lucas regulator from within it’s metal shell and fit the new regulator in there instead. The modern regulators are very small (about the size of a small matchbox) and so will easily fit. This way the bike still looks completely original (unless you take off the regulator cover of course!) and the new regulator gets some degree of weather protection (although they are sealed units anyway). Running the cables should also be quite easy since the existing bike wiring still only has to go to the same place. However, I didn’t want to risk damaging the old regulator trying to take it out of it’s shell and I preferred to leave this in place so the bike was still as physically complete and original as possible, even if the old Lucas unit wasn’t actually doing anything.

Another alternative would be to fit the new regulator within one of the two toolboxes which sit one on either side of the bike. This way they would again be hidden from sight, but fitting them here would involve drilling holes in the steel cases for the cables and fixing bolt to go through. I really wanted to avoid drilling holes in my bike as much as possible though.

I toyed with the idea of fitting the regulator onto the under side of the seat somewhere for a while. The seat frame I have is not original and already in not the best of conditions, so a few more holes wouldn’t be an issue. But then I decided that needing to disconnect the regulator every time I needed to remove the seat would be just too much hassle.

The new regulator mounted on the back of the old one

And so I finally decided that I would fit the new voltage regulator onto the back of the old regulator using the same mounting bracket. The bracket was already removed from the bike in order to inspect the old regulator, so it was a simple job to drill a single 5mm hole to mount the new unit on it’s rear. In this way the new regulator would be pretty much hidden from view by the old regulator unit which would still be in place, and the existing wiring would still be going to about the right place to connect it. It would also be fairly well protected from the elements by the seat, but still open to some airflow for cooling in case in gets warm in heavy usage.

Step 4 – Wiring in the new regulator

With the new (and old) regulators bolted back to the bike, the wiring connections could then be made. Like the old Lucas regulator, the DVR2 unit only has four wires (remember “FADE” – Field, Ammeter, Dynamo and Earth). Making the wiring connections is therefore fairly straightforward as long as you make sure that you get the wires the right way around. Double check everything as any wrong connections could damage your dynamo or new regulator unit (although the DVR2 is allegedly protected from most mis-connections).

Preparing the wiring for the new regulator

Actually, the DVR2 unit I’m fitting has one extra connection to those I mentioned above giving a total of five wires coming out of this little black (sorry silver) box. This fifth wire allows you to select between 6 volt or 12 volt operation modes.

The 12v mode squeezes enough extra volts out of your existing 6v dynamo to allow use of a 12v battery and bulbs without requiring any expensive dynamo armature replacements or rewindings. The downside is that you need a slightly higher engine speed before the dynamo is producing enough volts for the regulator to ‘cut in’.

If you connect the brown and white wires from the regulator together to the ammeter then you get standard 6v operation, if you do not connect the white (only the brown) then you get 12v operation (the white wire is just taped up out of the way). Simple hey! For now though I only needed 6 volts as I wanted to keep everything fairly standard.

The full data sheet and wiring diagram for the DVR2 regulator unit can be downloaded for reference from the manufacturers website here.

UPDATE: My old 6 volt battery recently failed and I wasn’t able to source a replacement locally. So I took the opportunity to switch my DVR2 across to 12 volt operation after fitting a new 12v battery and different bulbs. My guide explaining exactly how to go about this is here.

Step 5 – Making the connections

The new voltage regulator unit was supplied with brass bullet connectors on the ends of each of the five wires, but the existing bike had just bare wires where I had unscrewed them from the old regulator terminals.

The new regulator connection terminals

Searching through my toolbox I found a supply of matching brass bullet connectors (the same as fitted to the new regulator) and also some insulated male-to-male connectors. So I got to work with the soldering iron.

It’s worth making sure that all of these connections are as good as you can get as all of the current from the charging system will need to pass through them. Dodgy connections means extra resistance and hence less power (volts) getting from the dynamo to the battery and lights etc.

The three wires shown in the above photos are from the dynamo, field and ammeter connections but note that my bike has recently been rewired and so the colours are not original and won’t match your bike! Two wires from the DVR2 unit will connect to the ammeter wire, one to the dynamo and one to the field (the second connection on the dynamo).

The fifth wire is the earth wire which needs a good connection to the bike frame somewhere – I used a ring terminal onto the back of one of the regulator bracket bolts. Note that the regulator will need to be wired differently depending upon whether your bike is wired positive or negative earth (i.e. which terminal of the battery is connected to the frame). Pay close attention to this as fitting it the wrong way round could damage the dynamo or regulator unit.

Step 6 – Adding a fuse

No fuse was fitted as standard in the electrical systems of many old bikes, but they are a very, very, very good idea! I’ve lost track of the number of times I’ve accidentally shorted a live terminal of the ammeter, battery, etc to the frame with a screwdriver or the like whilst tinkering with something. A simple (correctly rated) fuse should stop any damage to the bike if you do something silly or if something goes wrong. So whilst you’re fitting the new regulator it’s a good idea to wire a fuse holder into the circuit as well. But don’t put the fuse in just yet though until all the connections are made, double checked and the battery reconnected.

There are two places that the manufacturer recommends fitting a fuse when installing the DVR2 regulator. The first is inline with the output (ammeter) wire from the regulator and the second is in the battery ‘live’ side of the ammeter. They recommend a 13 Amp fuse for 6v and a 10 Amp fuse for 12v operation.

The new regulator hidden away behind the original one

At the moment I have only fitted a single fuse between the battery’s positive terminal and the frame (my bike is wired positive earth).

Technically it should probably be on the live (i.e. negative side), but the wires of the fuse holder I had were red and connecting this to the negative (black) side of the battery was bound to add confusion later down the line. Realistically though, either side of the battery is fine.

I’ll probably add a second fuse between the regulator and ammeter later on when I get a second fuse holder though, just to be sure.

The blade fuses I have are available in 10 or 15 Amp ratings (not a 13 Amp as recommended) so I went with the 15 Amp. My dynamo is a small Lucas E3NL type rated at about 48 Watts output which at 6v would equate to 8 Amp current (48W / 6v = 8A). There’s sometimes a surge of current when you first turn on electrical items and so that you don’t keep blowing fuses in normal operation you need to slightly over-rate the fuse.

However the lower the fuse rating you can get away with the better as this will usually blow sooner (or for smaller electrical faults). I had no problem running with a 15 Amp fuse, and I suspect that a 10 Amp fuse would also have been ok in my system. Work out what fuse rating would be most suitable for your bike based upon the dynamo power rating and total electrical load of all the bulbs etc. If you’re not sure, it’s better to start small and work up (if the fuse blows in normal use), rather than fit a fuse with too high a current rating to start with.

A good starting point is to over-rate the fuse by 50% on top of whatever power you have calculated from adding up all of the bulb wattages etc. So in my case, 50% extra on top of my calculated 8A maximum current (from the dynamo) would mean I should have fitted a 12A fuse. But with no 12A fuse available, I chose to go up to 15A, rather than down to 10A.

The reason for over-rating the fuses is that some electrical components draw more current initially when they are cold. A cold bulb filament has less resistance than a hot one, so as the filament lights up and glows hot, the amount of current flowing through it decreases. Or to put it the other way round, a cold bulb draws more current than its rated power as the resistance of the filament is less. This extra current comes from the combination of the outputs from the battery and dynamo, so you can’t really just base your calculations on what power output the dynamo is supposed to give. Basically you’ll need to suck-it-and-see to a certain extent, just remember to start with low amp fuses and work up, not the other way round.

Step 6 – Checking and testing

With everything wired up it’s a good idea to make doubly sure that all the wires are connected correctly. Have another check that the wires to the ammeter, dynamo and field are all the right way around and use a multimeter to check for continuity if there is any doubt. The new regulator should have protection from most wiring faults, but best not take any chances and risk frying your new little black box or damaging your dynamo.

The new regulator installed

When you’re happy, replace and reconnect the battery and then when everything else is complete, install the fuse into the fuse holder.

And now the moment of truth! Firstly, before starting the engine, check that all the lights are working as they should. If they aren’t, or the new fuse you have fitted blows, then something is wrong and you should promptly disconnect the battery and recheck everything before continuing.

If all the lights seem to be working fine, turn them off and start the engine. Once it’s warm and happily ticking over you can then try all the lights again to make sure they are still working. Keep an eye on the ammeter. With the engine just ticking over (or stopped) and the headlight on there should be a reasonable discharge from the battery (i.e. the ammeter needle should move to the left). With the lights off, the needle should be about in the centre of the gauge indicating that not much charge is going into or out of the battery.

As you gradually increase the engine revs, the ammeter needle should slowly move to the right indicating that the dynamo is putting power into battery (i.e. the battery is being charged). How much the needle moves to the right will depend upon the state of charge of the battery; a full battery will only take a small ‘trickle’ current (say less than 1 Amp) whereas a flat battery might take the full output from the dynamo (perhaps 5+ Amps).

With the headlights on and at moderate revs, the output from the dynamo should just about balance the drain through the bulbs so that the needle is around the middle of the gauge again. The exact position will depend upon what power bulbs you have fitted; even the best charging system is going to have difficultly balancing and 60 Watt halogen monster!

You can also check the output of the dynamo and new regulator using a voltmeter connected across the battery terminals. With the engine off, a good 6v battery should show between about 6.1 to 6.3 volts. With the engine running and at a reasonably quick tickover, this voltage should rise to about 7.2 volts. This is the optimum charging voltage for a 6v battery – you have to have a few extra volts to overcome the internal resistances of the cells. More volts than this is certainly not better and could easily ‘boil’ the battery causing damage and possibly even cause an explosion!

For 12 volt systems, a good battery should give between about 12.4 to 12.6 volts output when fully charged and not under load. The optimum charging voltage is around 14.4v.

If everything seems well, it’s time for a test drive. Pay attention to the ammeter as you go along and see what charge is going into (or out of) the battery. If you turn on the headlight whilst stationary it should a significant discharge, but as you pull away and engine speed increases the regulator will start to feed current from the dynamo into the system. At a reasonable cruising speed (not at tickover or when racing the engine), the input from the dynamo and regulator should just about balance the output by the lights and so the ammeter needle should be somewhere near the middle.

Turn off all the lights and observe the ammeter again on a longer run. If the battery is partially discharged then the ammeter needle should move to the right indicating that current is going into the battery (i.e. to charge it). However, as the battery nears full charge the amount of current it accepts should drop and the ammeter reading should fall to just a small positive ‘trickle’ charge.

If it stays high after a long ride it’s probably an indication that there is something wrong with your battery as it is not taking the charge. Check the fluid levels (if it’s an unsealed lead-acid battery) and perhaps try charging with a good mains charger to see if that helps. Some modern chargers have built in battery ‘recovery’ functions which can help to bring some aging batteries back to life.

Conclusions

And that’s about it – a new regulator successfully installed and hopefully many more miles of happy riding!

I’d just like to point out that I have no connection whatsoever to any of the companies or products I have mentioned, other than in the case of the DVR2 regulator product having fitted one to my Matchless and been quite pleased with the results. Please make up your own mind about the suitably (or otherwise) of any products or services for your own requirements as these may be different to mine.

A friendly disclaimer in case you do something silly!

This article and website are based upon my own opinions, experiences, mistakes and successes and is provided for general interest and information purposes only. I hope you find it useful, but please do not rely blindly on anything I have written or done myself. You and your bike are different to me and my bike and so other considerations may apply. Any damage you inadvertently do to yourself or your bike is solely your own fault; I cannot be held responsible for your actions! The full terms and conditions for my website can be found here.

20 Responses to Replacing the regulator unit

Peter Grimadell20th September 2011

Just to say thanks for the guide on Lucas dynamo testing. I have a 1938 Sunbeam with a lucas magdyno and I suspected the dyno bit was doing nothing. Electrics are not my greatest strength and I had no idea how to test the dynamo. Using your guide I managed to establish there was no output and the likely cause was bushes. This proved to be the case and all is now well. Thanks again for a well written and detailed document.

Question for you: I have a ’59 Panther M120 with Lucas dyno, wired for negative Earth, and bought the DVR2 regulator to replace my Lucas Lucas MCR-2 regulator. I connected up the DVR as you and the DVR directions instructed. I have not started the engine yet, as noticed that when I turn the lights on the ammeter now dips to the + side instead of the – side as it did before. I used your instructions to flash the dyno from the Field to the battery + side, got the sparks as you’d expect, but see the same behavior. I can only assume that the ammeter is wired backwards, but this behavior didn’t occur with the old Lucas regulator.

Sounds like you might have your wires crossed! Your ammeter wires that is

Flashing the dynamo will have no effect until the engine is running, although you should check that it is correctly polarised for negative earth. Remember that my bike is positive earth and so any pictures or wiring I show or mention may need to be reversed for your negative earth system.

Presumably you bought a negative earth DVR2 unit, didn’t you? They come in both positive and negative earth varieties and you need the correct one as they can’t be changed round.

As long as that all seems ok then it would seem like the ammeter is wired round backwards wouldn’t it? Maybe a couple of wires got swapped around when the regulator was changed. If everything seems to work ok then nothing can be too wrong with the wiring. Start the engine and see if the ammeter moves the other way compared to when it is stopped with the headlights on. Keep rev’s low to start with (low voltage / current output) so you have time to spot anything wrong before causing any damage. The DVR2 unit is apparently well protected from incorrect wiring though so you should be ok.

Thanks for the tips, James. I took everything back apart and double checked that I’d got all the connections right, then I decided to reverse the wires on the Ammeter to make it respond correctly. I am sure that the previous owner must have wired it backwards. Surprise – I do have a fuse installed, it was there already when I got the bike!

Hi James, As I said before….great site (blog). An inspiration for me as an owner of a Matchless G3L ’49 with an ’51 engine.

I got a working dynamo Lucas EN3 on it and an working regulator (ugly thing) and want to replace it by an MCR2 L 6v with an beuatifull chrome cap. So it’s not a replacement as your’s for an modern one, but still I can use a lot of your information as above.

Cheers, Eric Heijdens
Thanks!

Another question: I’m searchin for an complete brake rod for my Matchless but can’t find one. Do you have any idea where I can get one? Sorry if my English is bad.

Hi Vijay. I’m not familiar with your motorcycle model, but if it currently uses a 6v Mcr2 regulator then the DRV2 should work just fine. Why don’t you contact the guys over at Dynamo Regulators to confirm? James

Hi James – A great article and pictures accompanied by clear instructions are excellent. I’ve just fitted the same unit to my positive earth 6V ’54 AJS Model 16 following your guide. I fitted the fuse and after checking everything a third time fired the bike up. Thankfully nothing went bang and with no lights the ammeter needle nestled in the middle. However a slight increase in the revs failed to move the needle much at all. When I switched the lights on the needle showed a considerable discharge but increasing revs didn’t bring about the expected needle swing to the right that I expected. However the lights stayed brightish despite the needle showing the discharge, and slight revs maintained their brightness. I am assuming the battery is fully charged and in good order so once it becomes discharged the ammeter will show a much higher level of charge. Any advice much appreciated. Cheers and Thanks – Dave

Hi Dave. Glad you found this useful and you went with the DVR2 – I’m sure it will serve you well for many years to come! I think you’re probably right with regards to the battery being fully charged, hence the reason the ammeter doesn’t show much of a charge. I should probably update the article at some point to make that clearer; it was originally written when my bike had a duff battery installed, so I also had a large swing to the right on the ammeter as it never reached a full charge. Let us know how you get on with the new reg. James 🙂

First of all, thank you for making a “do it yourself” guide, and making it very easy to understand. Its been a great help so far.
I have tried as much as possible, but now I am running out of pretended skills that I clearly doesn’t have.

I am working on an AJS 18S 6V pos earth (at least now it is) – but no batt was on the bike at purchase (from a barn – previous owner passed away)
So far I have repolarised my dynamo,
Changed the vintage regulator unit to a WESSELL 6 V pos earth,
Made the light bulb test on the dynamo (load test) to see if theres amps available, the with the connected D/F terminals through a 12V light bulb to fram – worked fine.
There is still no charge on amp meter.
Lights on = minus 6-7 amps on the amp meter. Then starting engine and rewing – still no charge/change.
Then measure on battery (new) 5.95V with engine on & off.
Lights off 6,29 volts, with engine on & off
Checked everything i can think of for bad connections

Interchanging the F & D terminals – no change. Do you Think destroyd anything by trying this
There are fuses between Battery (+) and frame & fuse between Wessell unit black and amp. meter

Rechecked the connections of the Wessell unit:

regulator yellow to dynamo D output;
regulator green to dynamo F field input;
regulator red to earth;
regulator black to ammeter (fuse) and a wire connection to the main light switch (without this one theres no light on the bike)
battery negative to ammeter
battery positive to earth (fuse)

Still no charge..

What would be your guess in this situation?
Disconnect all wires and make a charge test directly to a battery, and if charge is found, then work forward connecting the rest wire by wire?
Could it be something strange in the dynamo thats needs to be switched around?
I have no clue by now.

hi I have just had a battery blowout. Top blow off one side blow away. I purchased a new battery, fitted it after a few checks with a meter and the bike ran again. The bike is a 1958 Norton 99. The only problem I have now is the charging voltage is 14.73 this is a bit high any ideas anyone.
I love the clueless site but as a classic bike rider everyone has to know quiet a bit just to keep a 50year old bike on the road.

Hi Richard. As you say, 14.73 volts is a little on the high side; anything above about 14.4 volts (see Battery charging voltages) will result in the cells ‘gassing’ and, if the pressures builds up, the battery can blow, as you have found out! Also, was this voltage measured at tick-over, or could it be even higher when you’re riding along?

Either way, it sounds like your regulator is not functioning correctly. If it is the original mechanical one then it may be possible to adjust it. If it is a modern solid state unit then it may need replacing. I thoroughly recommend the DVR2 regulators.

Either way, I would suggest fixing the voltage issue before riding your bike again (or at least keep the headlamp at all times to burn off some of the excess voltage). Otherwise you risk damaging your new battery.

Hi James,
A great write up, by using your write up on “replacing the regulator” I have discovered that the electronic regulator on my AJS M20 is not giving a feed to the battery so looks like a new one requires fitting.
Do you have any hints on the following observation, I rebuilt the 6 volt Dynamo and it checks out fine however, when checking the output voltage using the jumper connecting F&D terminals at the dynamo I get 5 to 8 volts at tickover when I increase the revs it outputs 10 to 12 volts pluss !. I did replace the Armature and field windings with a 6 volt kit.
Will the increase voltage output damage the dynamo once the new regulator is fitted?

Hi James, great article, exactly what I’m looking for. I have a B31 which needs a new regulator and this is the perfect answer. I assume the fixing details are similar and is there anything I should be looking out for.